1. Field of the Invention
This invention is directed to a system for measuring workpieces and, more particularly, to an expandable ruler system that includes additional measuring and drafting functionality.
2. Background
"Measure twice, cut once," is a well-known axiom in woodworking. The usefulness and necessity of the phrase are derived from the difficulty associated with accurately measuring multiple workpieces during a project. In woodworking, accuracy is crucial. A wooden chair, for example, may require the precise cutting of several dozen parts. If one leg of the chair is shorter than another, the mistake will be quite noticeable. Rather than discovering the mistake during assembly, the prudent woodworker "measures twice" to ensure that the cut will be made at the correct location. He may then proceed to "cut once" and thereby avoid the wasteful procedure of cutting twice (or more).
Measurement errors also add up very quickly. A miter joint, one of the most commonly used joints, brings together two pieces of wood cut at respective 45 degree angles. When combined, the outer and inner edges of each piece form 90 degree angles to each other. Miter joints are often used in constructing picture frames and furniture edge-banding. If one piece of a four-sided picture frame is inaccurately measured, the resulting frame will have a gap at one or more of its joints. If all four pieces are improperly measured, the sum of the inaccuracies can be seen across each joint in the frame. Usually, the woodworker's only solution is to toss the entire frame and start over.
Measurement errors occur for a number of reasons. Many measurement errors may be associated with the measuring tools, themselves. Tape measures, the most common measuring tools, are well-known for their inaccuracy. Three tape measures from the same manufacturer and bearing the same model number are likely to provide three separate measurements differing by as much as 1/8 of an inch. The discrepancy between tape measures often lies in the steel hook at the end of the tape. The hook may be easily damaged by dropping the tape measure (a common occurrence) or by bending the hook during the measurement process. The tape measure also suffers from the stress applied to the tape by bending and turning the tape to obtain a measurement across several dimensions. Over time, the tape loses its form and depressions in the rule drawn across the tape cause inaccuracies in the tape measure.
Other measuring tools pose additional problems. A standard plastic or wood ruler can easily chip, thus resulting in the loss of measuring units. Moreover, the materials of such rulers are not sufficient to withstand the abuse that may be inflicted upon them in a workshop. Similarly, wooden folding rules are prone to inaccuracy due to mistreatment and the construction of the tool. Eventually, the folding mechanisms of the rules wear, thus making the rule both inaccurate and difficult to manage. Moreover, the hook at the end of the rule may be damaged like the hook on the tape measure.
One might assume that these problems might be avoided by using finer measurements (e.g., 1/64"). Unfortunately, most tape measures and folding rules do not include graduations finer than 1/16 of an inch. Fine measurements are usually found on more precise tools, such as combination squares and rules. But these tools typically are not long enough to measure boards longer than one foot. Two foot rules are usually considered too long to handle properly. Furthermore, the addition of finer graduations usually requires the elimination of metric units (i.e., one edge carries units in 1/4" and the opposite edge carries units in 1/32"). Alternatively, the woodworker can use multiple measuring devices having coarse units, fine units, and metric units. This can become quite unwieldy, however.
Some measurement inaccuracies occur due to human error in using the tools. The units on many tape measures and rules are very difficult to read under the heavy light commonly found in woodworking shops. Very often, the material used in constructing the tool causes a glare when the tool is viewed from certain angles. Thus, while the tool itself may be accurate, the woodworker reads it incorrectly leading to incorrect machining of the workpiece. Another common form of human error occurs when reading from the one inch mark. To get around the problems discussed above regarding banged-up hooks on tape measures, many woodworkers simply take the reading from the one inch mark. Unfortunately, they later forget to add an inch when taking the actual measurement. As a result, their workpieces end up one inch short.
Another problem associated with use of the English or Imperial measuring system is the need to remember at least three numbers when dealing with fractions of an inch. For example a measurement of 13 and 15/16ths inches may get miss-remembered or transposed as 15 and 13/16ths.
To eliminate many of the problems associated with measuring tools and human error, many woodworkers turn to "storyboards." A storyboard is a piece of wood that includes well-inscribed, commonly-used measurements. For example, if a project calls for twenty-four mortises (a "mortise" is a "hole" formed in or through the workpiece to receive a protruding "tenon" formed in another workpiece) of four inches in length, the storyboard will include one mark to indicate the beginning of the mortise and one mark to indicate the end of the mortise. The area between the two marks may be identified as "4 inch mortise" or a similar designation. The length between the two marks becomes the "standard" length for all mortises throughout the project. Thus, certain inaccuracies will not be of much concern since the measurement is uniform throughout the project. All of the mortises may have a length of 3 31/32", but this is preferable to mortises of varying length.
Storyboards have drawbacks, however. First, constructing storyboards can be time-consuming. Rather than immediately embarking upon a project, the woodworker must first construct one or more storyboards for the project. Second, multiple measurements along a single storyboard can become confusing. The only method for differentiating between measurements is a small title written within or beneath the measurement. Third, the wooden board used for the storyboard may be nicked or scuffed. Similarly, the board may accidentally contact a cutting machine midway through the project. A new storyboard would not contain the exact measurements of the previous storyboard. Also, storyboards may not be used for certain measurements that require the use of a combination square or similar item, such as accurately drawing lines.
The storyboard, therefore, is not the panacea for improper measurements. It has drawbacks and must be used in combination with other measuring tools. As discussed above, however, the more tools that are introduced into a project, the greater likelihood that measurement errors occur.